In the field of seismic data processing, “4D-noise” is an expression used to describe non-repeatable noises recorded during seismic surveys that do not disappear when time-lapse seismic signals are compared. In deepwater seismic applications, 4D-noise can result from many sources, including imperfect repeats of acquisition geometry, differences in multiple energy due to changes in water velocities, seismic interference noise from other vessels, swell noise, etc. Likewise, 4D-noise onshore can result from seasonal changes in the shallow subsurface, time-varying receiver ghosts, etc.
In deepwater environments where seismic recordings are made using ocean bottom sensors such as nodes (OBN) or cables (OBC), it is possible to produce two independent images of the subsurface using the conventional up-going wave-field and the down-going wave-field, respectively, via the well-known technique of mirror migration. It is useful to think about how the 4D-signal and 4D-noise compare within these two types of images. The 4D-signals that result from changes in reservoir properties should be the same on both the up-going and down-going data once the signals have been corrected for differences in wavelets and effective fold. In contrast, 4D-noise behaves quite differently. For example, 4D noise due to multiple contamination as a result of differences in water velocities will arrive at different times relative to a horizon of interest on the up- and down-going wave-fields. Similar issues arise in any instance in which it is desirable to generate a single image from two independent but simultaneous images of the subsurface, namely the signal in both images will be the same but the noise in both images will be different.
One way of take advantage of the difference in noise signals is to stack the up- and down-going wave-fields, which effectively doubles the fold of the survey and statistically suppresses noises by a factor of 1/√{square root over (2)}. This method does not always give the desired level of enhancement, however and it remains desirable to provide an improved technique.